<PAGE>

                                                                   EXHIBIT 10.24

THIS NOTE HAS NOT BEEN REGISTERED WITH THE UNITED STATES SECURITIES AND EXCHANGE
COMMISSION OR THE SECURITIES ADMINISTRATOR OF ANY STATE PURSUANT TO AN EXEMPTION
FROM REGISTRATION UNDER REGULATION D PROMULGATED UNDER THE SECURITIES ACT OF
1933, AS AMENDED (THE "ACT"). THIS NOTE IS SUBJECT TO THE TERMS OF A NOTE
PURCHASE AGREEMENT DATED AS OF MARCH 26, 2003, AS AMENDED, AND MAY NOT BE
TRANSFERRED OR SOLD EXCEPT AS PROVIDED THEREIN AND AS PERMITTED UNDER THE ACT
PURSUANT TO REGISTRATION OR EXEMPTION THEREFROM.

                              NEOPROBE CORPORATION

                       8.5% SENIOR NOTE DUE JUNE 30, 2005

$250,000.00                                                        April 2, 2003
                                                   (as amended on March 8, 2004)

         FOR VALUE RECEIVED, the undersigned, NEOPROBE CORPORATION (herein
called the "COMPANY"), a corporation organized and existing under the laws of
the State of Delaware hereby promises to pay to David C. Bupp, or registered
assigns, the principal sum of $250,000.00 on June 30, 2005, with interest
(computed on the basis of a 360-day year of twelve 30-day months) on the unpaid
balance thereof at the rate of 8.5% per annum from the date hereof, payable
monthly in arrears, on the last day of each calendar month, commencing with the
calendar month next succeeding the date hereof, until the principal hereof shall
have become due and payable.

         Payments of principal of, and interest on this Note are to be made in
lawful money of the United States of America at the address of the holder of
this Note provided for receipt of notices under the Note Purchase Agreement
referred to below or, at the option of the holder of this Note, in immediately
available funds at any bank or other financial institution capable of receiving
immediately available funds designated by the holder of this Note.

         This Note has been issued pursuant, and is subject, to an 8.5% Senior
Secured Note Purchase Agreement, dated as of March 26, 2003, as the same may be
amended, modified or supplemented from time to time in accordance with the terms
thereof (the "NOTE PURCHASE AGREEMENT"), between the Company and the Purchaser
named therein and is entitled to the benefits thereof. Each holder of this Note
will be deemed, by its acceptance hereof, (a) to have agreed to all of the terms
of the Note Purchase Agreement and other agreements referenced therein, and (b)
to have made the representations and warranties set forth in Sections 4.2
through 4.8 of the Note Purchase Agreement. This Note is secured, among other
things, pursuant to the Security Agreement as therein defined, and may now or
hereafter be secured by one or more other security agreements, mortgages, deeds
of trust, assignments or other instruments or agreements.

         This Note may be prepaid at any time prior to maturity in whole or in
part without premium or penalty.

         If an Event of Default, as defined in the Note Purchase Agreement,
occurs and is continuing, the principal of this Note may be declared or
otherwise become due and payable in the manner, at the price and with the effect
provided in the Note Purchase Agreement.

         This Note will be construed and enforced in accordance with and
governed by the laws of the

                                Page 1 of 2 pages

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                                                                   EXHIBIT 10.24

State of Ohio, without reference to principles of
conflicts of law. Any controversy, claim or dispute arising out of or relating
to this Note or the breach, termination, enforceability or validity of this
Note, including the determination of the scope or applicability of the agreement
to arbitrate set forth in this paragraph shall be determined exclusively by
binding arbitration in the City of Columbus, Ohio. The arbitration shall be
governed by the rules and procedures of the American Arbitration Association
(the "AAA") under its Commercial Arbitration Rules and its Supplementary
Procedures for Large, Complex Disputes; provided that persons eligible to be
selected as arbitrators shall be limited to attorneys-at-law each of whom (a) is
on the AAA's Large, Complex Case Panel or a Center for Public Resources ("CPR")
Panel of Distinguished Neutrals, or has professional credentials comparable to
those of the attorneys listed on such AAA and CPR Panels, and (b) has actively
practiced law (in private or corporate practice or as a member of the judiciary)
for at least 15 years in the State of Ohio concentrating in either general
commercial litigation or general corporate and commercial matters. Any
arbitration proceeding shall be before one arbitrator mutually agreed to by the
parties to such proceeding (who shall have the credentials set forth above) or,
if the parties are unable to agree to the arbitrator within 15 business days of
the initiation of the arbitration proceedings, then by the AAA. No provision of,
nor the exercise of any rights under, this paragraph shall limit the right of
any party to request and obtain from a court of competent jurisdiction in the
State of Ohio, County of Franklin (which shall have exclusive jurisdiction for
purposes of this paragraph) before, during or after the pendency of any
arbitration, provisional or ancillary remedies and relief including injunctive
or mandatory relief or the appointment of a receiver. The institution and
maintenance of an action or judicial proceeding for, or pursuit of, provisional
or ancillary remedies shall not constitute a waiver of the right of any party,
even if it is the plaintiff, to submit the dispute to arbitration if such party
would otherwise have such right. Each of the parties hereby submits
unconditionally to the exclusive jurisdiction of the state and federal courts
located in the County of Franklin, State of Ohio for purposes of this provision,
waives objection to the venue of any proceeding in any such court or that any
such court provides an inconvenient forum and consents to the service of process
upon it in connection with any proceeding instituted under this paragraph in the
same manner as provided for the giving of notice under the Note Purchase
Agreement. Judgment upon the award rendered may be entered in any court having
jurisdiction. The parties hereby expressly consent to the nonexclusive
jurisdiction of the state and federal courts situated in the County of Franklin,
State of Ohio for this purpose and waive objection to the venue of any
proceeding in such court or that such court provides an inconvenient forum. The
arbitrator shall have the power to award recovery of all costs (including
attorneys' fees, administrative fees, arbitrators' fees and court costs) to the
prevailing party. The arbitrator shall not have power, by award or otherwise, to
vary any of the provisions of this Note.

                                         NEOPROBE CORPORATION

                                         By     /s/ Brent L. Larson
                                           -------------------------------------
                                            Brent L. Larson, Vice President
                                            of Finance

                                Page 2 of 2 pages